Technical Abstract:
Expression of soybean aphid (SA) resistance was characterized among 496 soybean lines in a twice-replicated field-plot test at the Eastern South Dakota Soil and Water Research Farm near Brookings, SD, in 2009. Natural infestations of SA occurred but were supplemented by placing individual stems of infested colony plants into the upper canopy of experimental plants on July 28. Experimental plants were infested with SA obtained from a virus-free, multiclonal stock colony reared on the aphid-susceptible soybean line Pioneer 91B91 at NCARL. Stems used to infest field plots averaged 456 and 603 SA respectively for each replicate. Soybean infestations were allowed to progress through the summer. Test lines were rated twice by two persons using a scale ranging from 1 to 5, where 1 = 0 to 25 aphids per plant; 2 = 26 to 100 aphids per plant; 3 = 101 to 500 aphids per plant; 4 = 501 to 3,000 aphids per plant; and 5 = more than 3000 aphids per plant. Plants in replicate 1 were rated on 25 Aug and 31 Aug, and those in replicate 2 were rated on 26 Aug and Sep 2. Ratings were averaged across raters and dates. A line was considered resistant if its average rating was less than 3.0; moderately resistant, if its rating was from 3.0 to 3.9; and susceptible, if its rating was greater than 3.9. Six lines were resistant to SA, and these lines were aphid-resistant checks. Twelve lines were moderately resistant, including seven that have not been previously reported as aphid-resistant and five others that were also previously reported as aphid-resistant. The remainder of lines was susceptible.